CN114123325A - Converter control method and system for improving traditional protection action performance of electric power system - Google Patents

Converter control method and system for improving traditional protection action performance of electric power system Download PDF

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CN114123325A
CN114123325A CN202111406025.8A CN202111406025A CN114123325A CN 114123325 A CN114123325 A CN 114123325A CN 202111406025 A CN202111406025 A CN 202111406025A CN 114123325 A CN114123325 A CN 114123325A
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voltage
current
converter
fault
value
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CN114123325B (en
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梅红明
于华龙
王一
王继慷
刘树
张月品
张效宇
操丰梅
孙刚
徐可寒
杜兆强
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Beijing Sifang Automation Co Ltd
Beijing Sifang Engineering Co Ltd
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Beijing Sifang Automation Co Ltd
Beijing Sifang Engineering Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/50—Controlling the sharing of the out-of-phase component
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects

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  • Power Engineering (AREA)
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Abstract

The method for controlling the converter to improve the traditional protection action performance of the power system comprises the following steps: according to the voltage and the current of the grid-connected point of the converter, judging whether the alternating current power grid is in fault or normal state and whether the fault is cleared; based on active outer ring control and reactive outer ring control of the converter, obtaining an initial value of a converter inner ring control instruction current, calculating the converter instruction current under the fault or normal condition of an alternating current power grid, and performing closed-loop control on the converter current; obtaining the command voltage of the converter based on dead-beat control; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid. The method provided by the invention is suitable for wind power, photovoltaic, energy storage and other occasions, ensures that the converter does not overflow under various working conditions, simultaneously simulates the phase angle characteristics of the positive-sequence fault current and the negative-sequence fault current of the synchronous generator, obviously improves the relay protection action performance of the double-high power system, and ensures the stability of the double-high power system.

Description

Converter control method and system for improving traditional protection action performance of electric power system
Technical Field
The invention relates to the technical field of relay protection and voltage source converter control of a power system, in particular to a converter control method and system for improving the traditional protection action performance of the power system.
Background
The vigorous development of new energy sources, represented by photovoltaic and wind power, is an important way to achieve the "dual carbon" goal. The installed capacity of new energy in China exceeds 5.3 hundred million kilowatts and is the first place in the world. The requirements of scenes such as centralized new energy delivery, regional power grid interconnection, large-capacity long-distance power transmission and the like on high-voltage direct-current power transmission enable conventional and flexible direct currents to develop rapidly in China, and the direct power transmission scale exceeds 200 GW. In order to cope with the influence of the output fluctuation and intermittence of the new energy on the frequency quality and stability of the power system, the power system is expected to be accessed into the stored energy in a large scale in the future to improve the negative influence brought by the new energy. In addition, flexible direct current distribution is rapidly developed in recent years, and the duty ratio of power electronic type loads is increasing. Therefore, each link of the power system is full of a large number of power electronic devices, and modern power systems gradually form the trend of 'double high' of high-proportion new energy and 'high-proportion power electronic equipment'.
Whether a partial power converter represented by a double-fed wind power generation system or a full power converter represented by a photovoltaic system is obviously different from the fault characteristics of a synchronous generator. When the power grid fails, the synchronous generator shows a voltage source characteristic, and the converter shows a current source characteristic, so that the traditional relay protection cannot adapt to a double-high power system, and the problem of poor action performance is caused. For example, the distance protection rejection risk on the power electronics side increases significantly, and the current differential protection sensitivity decreases and even rejects in extreme cases. As a first line of defense of a power system, the operation performance of relay protection is extremely important to guarantee the safety of a power grid.
In prior art 1(CN104518525A) "protection control system and control method for ac/dc hybrid power grid power converter" aim at solving the output behavior of the grid-connected converter when the ac power grid side fails. The problems that the three-phase current of the system is unbalanced or non-sinusoidal, a large voltage ripple and power fluctuation exist on a direct current side and the like can be caused due to the existence of the negative sequence component of the power grid when the alternating current power grid fails, so that the safety of the converter is endangered, the service life of the converter is shortened, and the influence of the negative sequence component of the power grid on the system must be reasonably controlled. The adopted flexible positive sequence and negative sequence control strategy under the d-q synchronous coordinate system reasonably adjusts the positive and negative sequence contents of the alternating current network side current reference through the adjustment of the parameter variable, so that the network side current waveform quality, the system instantaneous active power and the system instantaneous reactive power are reasonably controlled, and the protection control of the converter is realized. However, although the application scenario is similar to that of a "dual high" power system, the method for controlling a converter during an ac fault is proposed in prior art 1, which aims to reasonably control the instantaneous power and solve the problems of dc lateral ripple and power fluctuation, but does not aim at improving the performance of ac protection operation.
Therefore, in order to improve the operation performance of the conventional relay protection in the context of the dual high power system, a new control strategy of the vsc needs to be researched.
Disclosure of Invention
In order to solve the defects in the prior art, the invention aims to provide a transformer winding deformation diagnosis method and system, which can be applied to various application occasions such as wind power, photovoltaic, energy storage and the like; the converter control method can ensure that the converter does not overcurrent under various working conditions, and can also simulate the phase angle characteristics of positive-sequence fault current and negative-sequence fault current of the synchronous generator, thereby remarkably improving the action performance of relay protection in the double-high power system and having great significance for ensuring the stability of the double-high power system.
The invention adopts the following technical scheme.
The converter control method for improving the traditional protection action performance of the power system comprises the following steps:
step 1, collecting voltage and current of a grid-connected point of a current transformer; carrying out positive-negative sequence separation and coordinate transformation on the voltage and the current to obtain positive sequence dq axis components and negative sequence dq axis components of the voltage and the current; obtaining a calculated value P of active power by using the dq axis components of the positive sequence and the negative sequencecalAnd a calculated value of reactive power Qcal；
Step 2, judging whether the alternating current power grid is in a fault state or a normal state according to the voltage and the current of the grid-connected point of the converter; when the fault state is judged, whether the fault is cleared or not needs to be judged;
step 3, acquiring an initial value of the instruction current of the converter inner loop control based on the active parameter outer loop control and the reactive parameter outer loop control of the converter;
step 4, respectively calculating the instruction current of the converter in the fault state or the normal state of the alternating current power grid based on the initial value of the instruction current, and carrying out closed-loop control on the current of the converter;
step 5, obtaining the instruction voltage of the converter based on dead-beat control by using the instruction current obtained in the step 4 and the voltage and current of the grid-connected point of the converter collected in the step 1; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid.
Preferably, step 1 comprises:
step 1.1, collecting three-phase alternating voltage u of grid-connected point of convertera、ub、ucAnd three-phase alternating current i of current transformer grid-connected point is collecteda、ib、ic(ii) a Extracting the angular frequency omega and the phase theta of three-phase positive sequence voltage from three-phase alternating voltage and current by a phase-locked loop;
step 1.2, 3s/2s conversion is carried out on the three-phase alternating voltage and the current to obtain an alpha beta component u of the three-phase alternating voltageα、uβAnd α β component i of three-phase alternating currentα、iβ(ii) a Wherein, three-phase alternating current voltage and current do 3s/2s transform, satisfy the following relational expression:
Figure BDA0003372261560000031
Figure BDA0003372261560000032
step 1.3, carrying out positive-negative sequence separation on the alpha beta components of the three-phase alternating voltage and the three-phase alternating current to obtain a positive sequence alpha beta component u of the three-phase alternating voltageαp、uβpNegative sequence alpha beta component u of three-phase AC voltageαn、uβnPositive sequence alpha beta component i of three-phase AC currentαp、iβpNegative sequence alpha beta component i of three-phase alternating currentαn、iβn(ii) a The positive and negative sequences of the alpha beta component of the three-phase alternating voltage and the current are separated, and the following relational expression is satisfied:
Figure BDA0003372261560000033
Figure BDA0003372261560000034
wherein F represents an electrical quantity, i.e. a voltage or current signal; f 'alpha and F' beta are electric quantity alpha beta component Fα、FβDelaying 1/4 the electrical quantity obtained during the power frequency period,
step 1.4, carrying out positive and negative sequence 2s/2r conversion on the positive and negative sequence alpha beta components of the three-phase alternating voltage and current to obtain a positive sequence dq component u of the three-phase alternating voltagedp、uqpNegative sequence dq component u of three-phase AC voltagedn、uqnPositive sequence dq component i of three-phase AC currentdp、iqpNegative sequence dq component i of three-phase alternating currentdn、iqn(ii) a The positive and negative sequence alpha beta components of the three-phase alternating voltage and the current are subjected to positive and negative sequence 2s/2r conversion, and the following relational expression is satisfied:
Figure BDA0003372261560000035
Figure BDA0003372261560000041
step 1.5, calculating active power and reactive power by using positive and negative sequence dq components of three-phase alternating voltage and current according to the following relational expression:
Figure BDA0003372261560000042
in the formula, PcalCalculated as active power, QcalThe reactive power calculation value is calculated.
Preferably, step 2 comprises:
step 2.1, collecting three-phase modulation wave signals e under normal operation of the power gridrefa、erefb、erefcRecord and save the currentThree-phase modulated wave signal e of previous whole period of sampling timerefa0、erefb0、erefc0；
Step 2.2, when the instantaneous value of the three-phase voltage is greater than the transient fixed value of the alternating current voltage, judging whether the voltage of the alternating current power grid is transient or not; otherwise, judging that the alternating current power grid is in a normal state;
and 2.3, when the voltage of the alternating current power grid is transient, detecting the state of the alternating current power grid according to the following criteria respectively:
1) when the positive sequence amplitude of the alternating current voltage is smaller than or equal to the low voltage fixed value, the positive sequence voltage of the alternating current power grid is judged to be too low;
2) when the positive sequence amplitude of the alternating current voltage is larger than or equal to the high voltage fixed value, judging that the positive sequence voltage of the alternating current power grid is too high;
3) when the difference value of the effective values of the three-phase voltages is larger than the unbalanced fixed value of the alternating-current voltage, judging that the alternating-current power grid voltage is unbalanced;
when the positive sequence voltage of the alternating current power grid is too low or the positive sequence voltage of the alternating current power grid is too high or the voltage of the alternating current power grid is unbalanced, judging that the alternating current power grid is in a fault state;
and 2.4, when the alternating current power grid is in a fault state, detecting the fault clearing state of the alternating current power grid according to the following criteria:
1) when the positive sequence amplitude of the alternating-current voltage is larger than the low-voltage fixed value and smaller than the high-voltage fixed value, the positive sequence voltage fault of the alternating-current power grid is judged to be cleared;
2) when the three-phase voltage effective value is smaller than the alternating voltage unbalance fixed value, the fact that the alternating current power grid voltage unbalance fault is cleared is judged;
3) when the angle difference between the negative sequence voltage and the negative sequence current is within the angle difference fixed value range before and after the fault is cleared, judging that the asymmetric fault of the alternating current power grid voltage is cleared;
when the AC power grid positive sequence voltage fault is cleared, the AC power grid voltage unbalance fault is cleared, and the AC power grid voltage asymmetry fault is cleared, the AC power grid fault is judged to be cleared.
Preferably, in step 2.2, the three-phase line voltage is instantaneousSubtracting instantaneous values of the same line voltage before 20ms from the values respectively, and when the maximum value of the absolute values of the phase deviation is greater than the transient fixed value delta U of the alternating voltage1If so, judging that the voltage of the alternating current power grid is transient, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the AC voltage transient constant value DeltaU1The value range is 0.08 pu-0.15 pu;
preferably, step 2.3 comprises:
step 2.3.1, when the positive sequence amplitude of the alternating voltage is less than or equal to the low voltage fixed value delta U2If so, judging that the positive sequence voltage of the alternating current power grid is too low, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the low voltage is constant value delta U2The value range is 0.8 pu-1.0 pu;
step 2.3.2, when the positive sequence amplitude of the alternating voltage is more than or equal to the high voltage fixed value delta U3If so, judging that the positive sequence voltage of the alternating current power grid is too high, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the high voltage is constant value delta U3The value range is 1.0 pu-1.2 pu;
step 2.3.3, subtracting the effective values of any two line voltages, wherein the absolute value of the difference is larger than the unbalanced constant value delta U of the alternating voltage4If so, judging that the voltage of the alternating current power grid is unbalanced, otherwise, judging that the voltage of the alternating current power grid is normal; wherein the constant value of the AC voltage unbalance is Delta U4The value range is 0.08 pu-0.15 pu.
Preferably, step 2.4 comprises:
step 2.4.1, the positive sequence amplitude of the alternating voltage is larger than the low-voltage fixed value delta U2And less than the high voltage constant value DeltaU3If the state duration is T, the positive sequence voltage fault of the alternating current power grid is judged to be cleared, otherwise, the positive sequence voltage fault of the alternating current power grid is judged not to be cleared; wherein the value range of the state duration T is 10 ms-30 ms;
step 2.4.2, subtracting the effective values of any two line voltages, wherein the maximum value of the absolute value of the difference value does not exceed the set value delta U of the unbalance of the alternating voltage4And if the state duration is T, determining that the voltage unbalance fault of the alternating current power grid is cleared, otherwise determiningPositive sequence voltage faults of the alternating current power grid are not cleared; wherein the value range of the state duration T is 10 ms-30 ms;
step 2.4.3, based on the determination result of step 2.4.2, when the angle difference between the negative sequence voltage and the negative sequence current before and after the fault is cleared is within the angle difference fixed value range, it is determined that the asymmetric fault of the alternating current power grid voltage is cleared, and the following relation is satisfied:
90°-x<∠In-∠Un<90°+x
in the formula (I), the compound is shown in the specification,
∠Inis the angle of the negative-sequence current,
∠Unis the angle of the negative-sequence voltage,
x is an angle criterion fixed value and the value range is 30-60 degrees.
Preferably, step 3 comprises:
step 3.1, calculating a value P by using active powercalAnd an active power target value PrefPerforming active power outer loop control on the converter;
step 3.2, utilizing the voltage measurement value U of the direct current sidedcAnd DC side voltage target value UdcrefPerforming outer ring control on the voltage of the direct current side of the converter;
step 3.3, calculating a value Q by utilizing the reactive powercalAnd a reactive power target value QrefPerforming reactive power outer loop control on the converter;
step 3.4, under the normal state of the alternating current power grid, the output current i 'controlled by the active outer ring'dprefAs d-axis positive sequence command current idprefInitial value of (1), output current i 'controlled by reactive power outer loop'qprefAs q-axis positive sequence command current iqprefIs started.
Preferably, in step 3.1, the converter active power outer loop control satisfies the following relation:
Figure BDA0003372261560000061
in the formula (I), the compound is shown in the specification,
PPIthe output signal of the PI controller, i.e. the active power control value,
Prefin order to be the active power target value,
Pcalthe power is calculated as the active power,
kp1for the scaling factor of the active power closed-loop control,
ki1the integral coefficient is the integral coefficient of active power closed-loop control;
s represents a differential operator;
in step 3.2, the outer ring control of the direct-current side voltage of the converter meets the following relational expression:
Figure BDA0003372261560000062
in the formula (I), the compound is shown in the specification,
UPIthe output signal of the PI controller, i.e. the dc voltage control value,
Udcrefis a target value of the voltage on the direct current side,
Udcis a measurement of the voltage on the dc side,
kp1for the scaling factor of the active power closed-loop control,
ki1the integral coefficient is the integral coefficient of active power closed-loop control;
in step 3.3, the converter reactive power outer loop control meets the following relational expression:
Figure BDA0003372261560000071
in the formula (I), the compound is shown in the specification,
QPIthe output signal of the PI controller, i.e. the reactive power control value,
Qrefin order to be the reactive power target value,
Qcalthe value is calculated for the reactive power,
kp2is a proportionality coefficient of the reactive power closed-loop control,
ki2is the integral coefficient of the reactive power closed-loop control.
Preferably, step 4 comprises:
step 4.1, setting the current direction flowing from the power grid to the converter as a positive direction;
step 4.2, in the normal state of the alternating current power grid, if the initial value of the d-axis positive sequence instruction current and the initial value of the q-axis positive sequence instruction current are both smaller than the maximum current I allowed to pass by the power electronic device in the convertermaxOutput current i 'controlled with the active outer loop'dprefAs d-axis positive sequence command current iqprefAnd output current i 'controlled by reactive power outer loop'qprefAs q-axis positive sequence command current iqprefAnd applying the dq axis negative sequence command current idnrefAnd iqnrefSetting the current values to be zero, and carrying out closed-loop control on the current of the converter;
if the initial value of the d-axis positive sequence command current or the initial value of the q-axis positive sequence command current is larger than the maximum current I allowed to pass by the power electronic device in the convertermaxThen, the dq-axis positive sequence command current i is calculated in the following relationdprefAnd iqprefAnd applying the dq axis negative sequence command current idnrefAnd iqnrefAll set to zero, carry out closed-loop control to the electric current of converter:
Figure BDA0003372261560000072
wherein σ is the output current i 'controlled by the active outer loop'qprefAnd output current i 'controlled by reactive power outer loop'qprefThe included angle between the composed resultant vectors satisfies
Figure BDA0003372261560000081
4.3, under the fault state of the alternating current power grid, calculating to obtain the fault instruction current of the converter by using the equivalent electromotive force before the fault and the voltage of a grid connection point during the fault; wherein the dq axis positive sequence fault command current idpref0And iqpref0Dq axis negative sequence fault command current idnref0And iqnref0Satisfies the following relation:
Figure BDA0003372261560000082
in the formula (I), the compound is shown in the specification,
udp、uqprespectively the dq-axis component of the positive sequence voltage of the grid-connected point during a fault,
udn、uqnrespectively the dq-axis component of the negative sequence voltage of the grid-connected point during a fault,
Lcis an equivalent inductance of the current transformer,
erefdp0、erefqp0respectively positive sequence dq axis components of the three-phase modulation wave signals of the power grid,
erefdn0、erefqn0respectively negative sequence dq axis components of the three-phase modulation wave signals of the power grid;
wherein, the three-phase modulation wave signal of the power grid is the three-phase modulation wave signal e of the previous whole period of the current sampling moment recorded and stored in the step 2.1refa0、erefb0、erefc0；
With the calculated dq-axis positive sequence command current idprefAnd iqprefDq axis negative sequence command current idnrefAnd iqnrefCarrying out closed-loop control on the current of the converter;
step 4.4, according to the fault instruction current obtained in the step 4.3, on the premise of keeping the phase of the fault instruction current unchanged, calculating the first coefficient K for reducing the fault instruction current in an equal proportion according to the following relational expressionref1：
Figure BDA0003372261560000083
When calculated Kref1If greater than 1, then K isref1Is set to 1;
calculating the fault command current equal-proportion reduction second coefficient K according to the following relationref2：
Figure BDA0003372261560000091
Pflt＝udpidpref0+uqpiqpref0+udnidnref0+uqniqnref0
In the formula, PnomThe active power output by the converter before the fault; pfltAlpha is a real number smaller than 1, and is the active power required to be output by the converter in the fault process;
get Kref1、Kref2The minimum value of the fault current is used as the equal proportional reduction coefficient K of the fault command currentrefThe reduced fault command current is calculated according to the following relation:
Figure BDA0003372261560000092
the fault command current after the equal proportion reduction can meet the condition that the fault command current is less than the maximum current I allowed by a power electronic device in the convertermaxThe requirements of (1).
Preferably, step 5 comprises:
step 5.1, 2r/3s conversion is carried out on the instruction current or the fault instruction current obtained in the step 4, and a positive sequence abc component i of the instruction current is obtainedparef、ipbref、ipcrefAnd negative sequence abc component inaref、inbref、incref；
Step 5.2, converting the direct current quantity into an alternating current quantity, namely obtaining a reference value i of three phases of the instruction current A, B, C according to the following relational expressionrefa、irefb、irefc：
Figure BDA0003372261560000093
Step 5.3, with command current irefa、irefb、irefcAnd step 1, collecting the voltage u of the grid-connected point of the convertera、ub、ucAnd current ia、ib、icObtaining a command voltage e of the converter based on dead-beat control as an input signalrefa、erefb、erefcThe following relational expression is satisfied:
Figure BDA0003372261560000094
in the formula: t issIs a control period; k is a radical ofDBThe beat-free coefficient is in a range of 0 to 1;
step 5.4, using the command voltage erefa、erefb、erefcThe three-phase duty ratio signal d is calculated in the following relationa、db、dc：
Figure BDA0003372261560000101
In the formula udcThe measured value of the voltage on the direct current side of the converter is obtained;
and 5.5, modulating the three-phase duty ratio signal to obtain trigger pulses of each power electronic device in the converter.
Promote converter control system of traditional protection action performance of electric power system, include: the power acquisition module, the power control module, the power grid state identification module and the converter control module;
the power acquisition module is used for acquiring the voltage and the current of a grid-connected point of the converter; the device is also used for carrying out positive-negative sequence separation and coordinate transformation on the voltage and the current so as to obtain positive sequence dq axis components and negative sequence dq axis components of the voltage and the current; obtaining an active power calculation value and a reactive power calculation value by utilizing the positive and negative sequence dq axis components;
the power control module is used for outputting an initial value of instruction current controlled by the inner ring of the converter based on the active parameter outer ring control and the reactive parameter outer ring control of the converter by taking the active power calculation value and the reactive power calculation value output by the power acquisition module as input data;
the power grid state identification module is used for judging whether the alternating current power grid is in a fault state or a normal state according to the voltage and the current of the grid-connected point of the converter; when the fault state is judged, whether the fault is cleared or not needs to be judged;
the converter control module is used for respectively calculating the instruction current of the converter in the fault state or the normal state of the alternating current power grid based on the initial value of the instruction current and carrying out closed-loop control on the current of the converter; the control circuit is also used for obtaining the instruction voltage of the converter based on the instruction current, the voltage and the current of the grid-connected point of the converter and based on dead-beat control; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid.
Compared with the prior art, the invention has the beneficial effects that:
1. the novel control strategy of the voltage source converter disclosed by the invention only changes the calculation mode of the current instruction, does not change the current inner loop control link, and is simple in strategy and easy to realize.
2. The control strategy disclosed by the invention can simulate the current sequence component phase angle characteristic of the synchronous generator on the premise that the current transformer does not overcurrent when the power grid fails, and can obviously improve the action performance of the traditional relay protection principle of the double-high power system.
3. The scheme disclosed by the invention is a key technology for ensuring the safe operation of a power grid by accessing new energy and power electronic equipment in a large scale, and is an important technical support for realizing a power system with the main body of the new energy and a double-carbon target.
Drawings
FIG. 1 is a block diagram of the steps of a converter control method for improving the performance of conventional protection actions of an electric power system according to the present invention;
fig. 2 is a schematic block diagram of current clipping and current command calculation according to an embodiment of the present invention.
Detailed Description
The present application is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present application is not limited thereby.
As shown in fig. 1, the converter control method for improving the performance of the conventional protection operation of the power system includes steps 1 to 5, which are as follows:
step 1, collecting voltage and current of a grid-connected point of a current transformer; carrying out positive-negative sequence separation and coordinate transformation on the voltage and the current to obtain positive sequence dq axis components and negative sequence dq axis components of the voltage and the current; obtaining a calculated value P of active power by using the dq axis components of the positive sequence and the negative sequencecalAnd a calculated value of reactive power Qcal。
Specifically, step 1 comprises:
step 1.1, collecting three-phase alternating voltage u of grid-connected point of convertera、ub、ucAnd three-phase alternating current i of current transformer grid-connected point is collecteda、ib、ic(ii) a Extracting the angular frequency omega and the phase theta of three-phase positive sequence voltage from three-phase alternating voltage and current by a phase-locked loop;
step 1.2, 3s/2s conversion is carried out on the three-phase alternating voltage and the current to obtain an alpha beta component u of the three-phase alternating voltageα、uβAnd α β component i of three-phase alternating currentα、uβ(ii) a Wherein, three-phase alternating voltage and current are converted for 3s/2s, and the three-phase alternating voltage and current satisfy relational expressions (1) and (2):
Figure BDA0003372261560000111
Figure BDA0003372261560000112
step 1.3, carrying out positive-negative sequence separation on the alpha beta components of the three-phase alternating voltage and the three-phase alternating current to obtain a positive sequence alpha beta component u of the three-phase alternating voltageαp、uβpNegative sequence alpha beta component u of three-phase AC voltageαn、uβnPositive sequence alpha beta component i of three-phase AC currentαp、iβpNegative sequence alpha beta component i of three-phase alternating currentαn、iβn(ii) a Wherein, the positive and negative sequences of the alpha beta component of the three-phase alternating voltage and the current are separated, and the relation formulas (3) and (4) are satisfied:
Figure BDA0003372261560000121
Figure BDA0003372261560000122
wherein F represents an electrical quantity, i.e. a voltage or current signal; f'α、F′βRespectively, the electric quantity alpha beta component Fα、FβDelaying 1/4 the electrical quantity obtained during the power frequency period,
step 1.4, carrying out positive and negative sequence 2s/2r conversion on the positive and negative sequence alpha beta components of the three-phase alternating voltage and current to obtain a positive sequence dq component u of the three-phase alternating voltagedp、uqpNegative sequence dq component u of three-phase AC voltagedn、uqnPositive sequence dq component i of three-phase AC currentdp、iqpNegative sequence dq component i of three-phase alternating currentdn、iqn(ii) a Wherein, the positive and negative sequence alpha beta components of the three-phase alternating voltage and current are subjected to positive and negative sequence 2s/2r conversion, and the following relational expressions (5) and (6) are satisfied:
Figure BDA0003372261560000123
Figure BDA0003372261560000124
step 1.5, calculating active power and reactive power by using positive and negative sequence dq components of three-phase alternating voltage and current according to a relation (7):
Figure BDA0003372261560000125
in the formula, PcalCalculated as active power, QcalThe reactive power calculation value is calculated.
Step 2, judging whether the alternating current power grid is in a fault state or a normal state according to the voltage and the current of the grid-connected point of the converter; when the fault state is determined, it is also necessary to determine whether the fault has been cleared.
Specifically, step 2 comprises:
step 2.1, collecting three-phase modulation wave signals e under normal operation of the power gridrefa、erefb、erefcRecording and storing three-phase modulation wave signal e of previous whole period of current sampling momentrefa0、erefb0、erefc0。
Step 2.2, when the instantaneous value of the three-phase voltage is greater than the transient fixed value of the alternating current voltage, judging whether the voltage of the alternating current power grid is transient or not; otherwise, the AC power grid is judged to be in a normal state.
Preferably, in step 2.2, the instantaneous values of the three-phase line voltages are subtracted from the instantaneous values of the same-name line voltages 20ms ago, respectively, when the maximum of the absolute values of the phase deviations is greater than the instantaneous value Δ U of the alternating voltage transient1If so, judging that the voltage of the alternating current power grid is transient, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the AC voltage transient constant value DeltaU1The value range is 0.08 pu-0.15 pu; in the preferred embodiment of the invention, the AC voltage transient set value delta U is determined according to experiments and field operation experience10.12pu was taken.
And 2.3, when the voltage of the alternating current power grid is transient, detecting the state of the alternating current power grid according to the following criteria respectively:
1) when the positive sequence amplitude of the alternating current voltage is smaller than or equal to the low voltage fixed value, the positive sequence voltage of the alternating current power grid is judged to be too low;
2) when the positive sequence amplitude of the alternating current voltage is larger than or equal to the high voltage fixed value, judging that the positive sequence voltage of the alternating current power grid is too high;
3) when the difference value of the effective values of the three-phase voltages is larger than the unbalanced fixed value of the alternating-current voltage, judging that the alternating-current power grid voltage is unbalanced;
and when the positive sequence voltage of the alternating current power grid is too low or the positive sequence voltage of the alternating current power grid is too high or the voltage of the alternating current power grid is unbalanced, judging that the alternating current power grid is in a fault state.
Preferably, step 2.3 comprises:
step 2.3.1, when the positive sequence amplitude of the alternating voltage is less than or equal to the low voltage fixed valueΔU2If so, judging that the positive sequence voltage of the alternating current power grid is too low, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the low voltage is constant value delta U2The value range is 0.8 pu-1.0 pu; in the preferred embodiment of the invention, the low voltage constant value delta U is determined according to experiments and field operation experience20.9pu was taken.
Step 2.3.2, when the positive sequence amplitude of the alternating voltage is more than or equal to the high voltage fixed value delta U3If so, judging that the positive sequence voltage of the alternating current power grid is too high, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the high voltage is constant value delta U3The value range is 1.0 pu-1.2 pu; in the preferred embodiment of the invention, the high voltage constant value delta U is determined according to experiments and field operation experience31.15pu was taken.
Step 2.3.3, subtracting the effective values of any two line voltages, wherein the absolute value of the difference is larger than the unbalanced constant value delta U of the alternating voltage4If so, judging that the voltage of the alternating current power grid is unbalanced, otherwise, judging that the voltage of the alternating current power grid is normal; wherein the constant value of the AC voltage unbalance is Delta U4The value range is 0.08 pu-0.15 pu; in the preferred embodiment of the invention, the fixed value delta U of the alternating voltage unbalance is determined according to experiments and field operation experience4Take 0.1 pu.
And 2.4, when the alternating current power grid is in a fault state, detecting the fault clearing state of the alternating current power grid according to the following criteria:
1) when the positive sequence amplitude of the alternating-current voltage is larger than the low-voltage fixed value and smaller than the high-voltage fixed value, the positive sequence voltage fault of the alternating-current power grid is judged to be cleared;
2) when the three-phase voltage effective value is smaller than the alternating voltage unbalance fixed value, the fact that the alternating current power grid voltage unbalance fault is cleared is judged;
3) when the angle difference between the negative sequence voltage and the negative sequence current is within the angle difference fixed value range before and after the fault is cleared, judging that the asymmetric fault of the alternating current power grid voltage is cleared;
when the AC power grid positive sequence voltage fault is cleared, the AC power grid voltage unbalance fault is cleared, and the AC power grid voltage asymmetry fault is cleared, the AC power grid fault is judged to be cleared.
Preferably, step 2.4 comprises:
step 2.4.1, the positive sequence amplitude of the alternating voltage is larger than the low-voltage fixed value delta U2And less than the high voltage constant value DeltaU3If the state duration is T, the positive sequence voltage fault of the alternating current power grid is judged to be cleared, otherwise, the positive sequence voltage fault of the alternating current power grid is judged not to be cleared; wherein the value range of the state duration T is 10 ms-30 ms; in the preferred embodiment of the present invention, the state duration T is determined to be 20ms based on experimental and field operating experience.
Step 2.4.2, subtracting the effective values of any two line voltages, wherein the maximum value of the absolute value of the difference value does not exceed the set value delta U of the unbalance of the alternating voltage4If the state duration is T, determining that the voltage unbalance fault of the alternating current power grid is cleared, otherwise determining that the positive sequence voltage fault of the alternating current power grid is not cleared; wherein the value range of the state duration T is 10 ms-30 ms; in the preferred embodiment of the present invention, the state duration T is determined to be 20ms based on experimental and field operating experience.
Step 2.4.3, based on the determination result of step 2.4.2, when the angle difference between the negative sequence voltage and the negative sequence current before and after the fault is cleared is within the angle difference fixed value range, determining that the asymmetric fault of the alternating current power grid voltage is cleared, and satisfying the relation (8):
90°-x<∠In-∠Un<90°+x…………(8)
in the formula (I), the compound is shown in the specification,
∠Inis the angle of the negative-sequence current,
∠Unis the angle of the negative-sequence voltage,
x is an angle criterion fixed value and the value range is 30-60 degrees; in the preferred embodiment of the invention, the fixed value x of the angle criterion is determined to be 45 degrees according to experiments and field operation experience.
And 3, acquiring an initial value of the instruction current of the inner ring control of the converter based on the outer ring control of the active parameter and the outer ring control of the reactive parameter of the converter.
Specifically, step 3 includes:
step 31, calculating a value P by using active powercalAnd an active power target value PrefPerforming active power outer loop control on the converter;
preferably, in step 3.1, the converter active power outer loop control satisfies the following relation:
Figure BDA0003372261560000151
in the formula (I), the compound is shown in the specification,
PPIthe output signal of the PI controller, i.e. the active power control value,
Prefin order to be the active power target value,
Pcalthe power is calculated as the active power,
kp1for the scaling factor of the active power closed-loop control,
ki1the integral coefficient is the integral coefficient of active power closed-loop control;
s denotes a differential operator.
Step 3.2, utilizing the voltage measurement value U of the direct current sidedcAnd DC side voltage target value UdcrefPerforming outer ring control on the voltage of the direct current side of the converter;
in step 3.2, the outer ring control of the direct-current side voltage of the converter meets the following relational expression:
Figure BDA0003372261560000152
in the formula (I), the compound is shown in the specification,
UPIthe output signal of the PI controller, i.e. the dc voltage control value,
Udcrefis a target value of the voltage on the direct current side,
Udcis a measurement of the voltage on the dc side,
kp1for the scaling factor of the active power closed-loop control,
ki1the integral coefficient of active power closed-loop control is adopted.
And 3. step 3.3, calculating a value Q by using the reactive powercalAnd a reactive power target value QrefAnd carrying out reactive power outer loop control on the converter.
In step 3.3, the converter reactive power outer loop control meets the following relational expression:
Figure BDA0003372261560000161
in the formula (I), the compound is shown in the specification,
QPIthe output signal of the PI controller, i.e. the reactive power control value,
Qrefin order to be the reactive power target value,
Qcalthe value is calculated for the reactive power,
kp2is a proportionality coefficient of the reactive power closed-loop control,
ki2is the integral coefficient of the reactive power closed-loop control.
Step 3.4, under the normal state of the alternating current power grid, the output current i 'controlled by the active outer ring'dprefAs d-axis positive sequence command current idprefInitial value of (1), output current i 'controlled by reactive power outer loop'qprefAs q-axis positive sequence command current iqprefIs started.
And 4, respectively calculating the instruction current of the converter in the fault state or the normal state of the alternating current power grid based on the initial value of the instruction current, and performing closed-loop control on the current of the converter.
Specifically, as shown in fig. 2, step 4 includes:
step 4.1, setting the current direction flowing from the power grid to the converter as a positive direction;
step 4.2, in the normal state of the alternating current power grid, if the initial value of the d-axis positive sequence instruction current and the initial value of the q-axis positive sequence instruction current are both smaller than the maximum current I allowed to pass by the power electronic device in the convertermaxOutput current i 'controlled with the active outer loop'dprefAs d-axis positive sequence command current idprefAnd output current i 'controlled by reactive power outer loop'qprefAs q-axis positive sequence instructionsCurrent iqprefAnd applying the dq axis negative sequence command current idnrefAnd iqnrefSetting the current values to be zero, and carrying out closed-loop control on the current of the converter; satisfies the relation (10):
Figure BDA0003372261560000162
if the initial value of the d-axis positive sequence command current or the initial value of the q-axis positive sequence command current is larger than the maximum current I allowed to pass by the power electronic device in the convertermaxThen, the dq-axis positive sequence command current i is calculated by the relation (11)dprefAnd iqprefAnd applying the dq axis negative sequence command current idnrefAnd iqnrefAll set to zero, carry out closed-loop control to the electric current of converter:
Figure BDA0003372261560000171
wherein σ is the output current i 'controlled by the active outer loop'dprefAnd output current i 'controlled by reactive power outer loop'qprefThe included angle between the composed resultant vectors satisfies
Figure BDA0003372261560000172
4.3, under the fault state of the alternating current power grid, calculating to obtain the fault instruction current of the converter by using the equivalent electromotive force before the fault and the voltage of a grid connection point during the fault; wherein the dq axis positive sequence fault command current idqref0And ipqref0Dq axis negative sequence fault command current idnref0And iqnref0Satisfies the relation (12):
Figure BDA0003372261560000173
in the formula (I), the compound is shown in the specification,
udp、uqprespectively the dq-axis component of the positive sequence voltage of the grid-connected point during a fault,
udn、uqnrespectively the dq-axis component of the negative sequence voltage of the grid-connected point during a fault,
Lcis an equivalent inductance of the current transformer,
erefdp0、erefqp0respectively positive sequence dq axis components of the three-phase modulation wave signals of the power grid,
erefdn0、erefqn0respectively negative sequence dq axis components of the three-phase modulation wave signals of the power grid;
wherein, the three-phase modulation wave signal of the power grid is the three-phase modulation wave signal e of the previous whole period of the current sampling moment recorded and stored in the step 2.1refa0、erefb0、erefc0；
With the calculated dq-axis positive sequence command current idprefAnd iqprefDq axis negative sequence command current idnrefAnd iqnrefCarrying out closed-loop control on the current of the converter;
step 4.4, according to the fault instruction current obtained in the step 4.3, on the premise of keeping the phase of the fault instruction current unchanged, calculating the fault instruction current equal-proportion reduction first coefficient K according to a relational expression (13)ref1：
Figure BDA0003372261560000181
When calculated Kref1If greater than 1, then K isref1Is set to 1;
calculating the fault command current equal proportional reduction second coefficient K by the relation (14)ref2：
Figure BDA0003372261560000182
And the active power P output by the converter is needed in the fault processfltSatisfies the relation (15):
Pflt＝udpidpref0+udpidpref0+udnidnref0+uqniqnref0…………(15)
in the formula, PnomThe active power output by the converter before the fault; pfltAlpha is a real number smaller than 1, and is the active power required to be output by the converter in the fault process;
get Kref1、Kref2The minimum value of the fault current is used as the equal proportional reduction coefficient K of the fault command currentrefSatisfy Kref＝min(Kref1,Kref2) The reduced fault command current is calculated by the relation (16):
Figure BDA0003372261560000183
the fault command current after the equal proportion reduction can meet the condition that the fault command current is less than the maximum current I allowed by a power electronic device in the converterOf maxAnd (4) requiring.
Step 5, obtaining the instruction voltage of the converter based on dead-beat control by using the instruction current obtained in the step 4 and the voltage and current of the grid-connected point of the converter collected in the step 1; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid.
Specifically, step 5 comprises:
step 5.1, 2r/3s conversion is carried out on the instruction current or the fault instruction current obtained in the step 4, and a positive sequence abc component i of the instruction current is obtainedparef、ipbref、ipcrefAnd negative sequence abc component inaref、inbref、incref；
Step 5.2, converting the direct current quantity into an alternating current quantity, namely obtaining a reference value i of three phases of the instruction current A, B, C according to a relation (17)refa、irefb、irefc：
Figure BDA0003372261560000191
Step 5.3, with command current irefa、irefb、irefcAnd step 1, collecting the voltage u of the grid-connected point of the convertera、ub、ucAnd current ia、ib、icObtaining a command voltage e of the converter based on dead-beat control as an input signalrefa、erefb、erefcSatisfies the relation (18):
Figure BDA0003372261560000192
in the formula: t issIs a control period; k is a radical ofDBThe beat-free coefficient is in a range of 0 to 1;
step 5.4, using the command voltage erefa、erefb、erefcCalculating the three-phase duty ratio signal d by the relation (19)a、db、dc：
Figure BDA0003372261560000193
In the formula udcThe measured value of the voltage on the direct current side of the converter is obtained;
and 5.5, modulating the three-phase duty ratio signal to obtain trigger pulses of each power electronic device in the converter.
Promote converter control system of traditional protection action performance of electric power system, include: the power acquisition module, the power control module, the power grid state identification module and the converter control module;
the power acquisition module is used for acquiring the voltage and the current of a grid-connected point of the converter; the device is also used for carrying out positive-negative sequence separation and coordinate transformation on the voltage and the current so as to obtain positive sequence dq axis components and negative sequence dq axis components of the voltage and the current; obtaining an active power calculation value and a reactive power calculation value by utilizing the positive and negative sequence dq axis components;
the power control module is used for outputting an initial value of instruction current controlled by the inner ring of the converter based on the active parameter outer ring control and the reactive parameter outer ring control of the converter by taking the active power calculation value and the reactive power calculation value output by the power acquisition module as input data;
the power grid state identification module is used for judging whether the alternating current power grid is in a fault state or a normal state according to the voltage and the current of the grid-connected point of the converter; when the fault state is judged, whether the fault is cleared or not needs to be judged;
the converter control module is used for respectively calculating the instruction current of the converter in the fault state or the normal state of the alternating current power grid based on the initial value of the instruction current and carrying out closed-loop control on the current of the converter; the control circuit is also used for obtaining the instruction voltage of the converter based on the instruction current, the voltage and the current of the grid-connected point of the converter and based on dead-beat control; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid.
The present applicant has described and illustrated embodiments of the present invention in detail with reference to the accompanying drawings, but it should be understood by those skilled in the art that the above embodiments are merely preferred embodiments of the present invention, and the detailed description is only for the purpose of helping the reader to better understand the spirit of the present invention, and not for limiting the scope of the present invention, and on the contrary, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention.

Claims (10)

1. The converter control method for improving the traditional protection action performance of the power system is characterized in that,
the method comprises the following steps:
step 1, collecting voltage and current of a grid-connected point of a current transformer; carrying out positive-negative sequence separation and coordinate transformation on the voltage and the current to obtain positive sequence dq axis components and negative sequence dq axis components of the voltage and the current; obtaining an active power calculation value and a reactive power calculation value by utilizing the positive and negative sequence dq axis components;
step 2, judging whether the alternating current power grid is in a fault state or a normal state according to the voltage and the current of the grid-connected point of the converter; when the fault state is judged, whether the fault is cleared or not needs to be judged;
step 3, acquiring an initial value of the instruction current of the converter inner loop control based on the active parameter outer loop control and the reactive parameter outer loop control of the converter;
step 4, respectively calculating the instruction current of the converter in the fault state or the normal state of the alternating current power grid based on the initial value of the instruction current, and carrying out closed-loop control on the current of the converter;
step 5, obtaining the instruction voltage of the converter based on dead-beat control by using the instruction current obtained in the step 4 and the voltage and current of the grid-connected point of the converter collected in the step 1; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid.
2. The method as claimed in claim 1, wherein the converter control method for improving the performance of conventional protection operation of the power system,
the step 1 comprises the following steps:
step 1.1, collecting three-phase alternating voltage u of grid-connected point of convertera、ub、ucAnd three-phase alternating current i of current transformer grid-connected point is collecteda、ib、ic(ii) a Extracting the angular frequency omega and the phase theta of three-phase positive sequence voltage from three-phase alternating voltage and current by a phase-locked loop;
step 1.2, 3s/2s conversion is carried out on the three-phase alternating voltage and the current to obtain an alpha beta component u of the three-phase alternating voltagea、uβAnd α β component i of three-phase alternating currenta、iβ(ii) a Wherein, three-phase alternating current voltage and current do 3s/2s transform, satisfy the following relational expression:
Figure FDA0003372261550000011
Figure FDA0003372261550000012
step 1.3, carrying out positive-negative sequence separation on the alpha beta components of the three-phase alternating voltage and the three-phase alternating current to obtain a positive sequence alpha beta component u of the three-phase alternating voltageαp、uβpNegative sequence alpha beta component u of three-phase AC voltageαn、uβnPositive sequence alpha beta component i of three-phase AC currentαp、iβpNegative sequence alpha beta component i of three-phase alternating currentαn、iβn(ii) a The positive and negative sequences of the alpha beta component of the three-phase alternating voltage and the current are separated, and the following relational expression is satisfied:
Figure FDA0003372261550000021
Figure FDA0003372261550000022
wherein F represents an electrical quantity, i.e. a voltage or current signal; f'α、F′βRespectively, the electric quantity alpha beta component Fα、FβDelaying 1/4 the electrical quantity obtained during the power frequency period,
step 1.4, carrying out positive and negative sequence 2s/2r conversion on the positive and negative sequence alpha beta components of the three-phase alternating voltage and current to obtain a positive sequence dq component u of the three-phase alternating voltagedp、uqpNegative sequence dq component u of three-phase AC voltagedn、uqnPositive sequence dq component i of three-phase AC currentdp、iqpNegative sequence dq component i of three-phase alternating currentdn、iqn(ii) a The positive and negative sequence alpha beta components of the three-phase alternating voltage and the current are subjected to positive and negative sequence 2s/2r conversion, and the following relational expression is satisfied:
Figure FDA0003372261550000023
Figure FDA0003372261550000024
step 1.5, calculating active power and reactive power by using positive and negative sequence dq components of three-phase alternating voltage and current according to the following relational expression:
Figure FDA0003372261550000025
in the formula, PcalCalculated as active power, QcalThe reactive power calculation value is calculated.
3. The method as claimed in claim 2, wherein the converter control method for improving the performance of conventional protection operation of the power system,
the step 2 comprises the following steps:
step 2.1, collecting three-phase modulation wave signals e under normal operation of the power gridrefa、erefb、erefcRecording and storing three-phase modulation wave signal e of previous whole period of current sampling momentrefa0、erefb0、erefc0；
Step 2.2, when the instantaneous value of the three-phase voltage is greater than the transient fixed value of the alternating current voltage, judging whether the voltage of the alternating current power grid is transient or not; otherwise, judging that the alternating current power grid is in a normal state;
and 2.3, when the voltage of the alternating current power grid is transient, detecting the state of the alternating current power grid according to the following criteria respectively:
1) when the positive sequence amplitude of the alternating current voltage is smaller than or equal to the low voltage fixed value, the positive sequence voltage of the alternating current power grid is judged to be too low;
2) when the positive sequence amplitude of the alternating current voltage is larger than or equal to the high voltage fixed value, judging that the positive sequence voltage of the alternating current power grid is too high;
3) when the difference value of the effective values of the three-phase voltages is larger than the unbalanced fixed value of the alternating-current voltage, judging that the alternating-current power grid voltage is unbalanced;
when the positive sequence voltage of the alternating current power grid is too low or the positive sequence voltage of the alternating current power grid is too high or the voltage of the alternating current power grid is unbalanced, judging that the alternating current power grid is in a fault state;
and 2.4, when the alternating current power grid is in a fault state, detecting the fault clearing state of the alternating current power grid according to the following criteria:
1) when the positive sequence amplitude of the alternating-current voltage is larger than the low-voltage fixed value and smaller than the high-voltage fixed value, the positive sequence voltage fault of the alternating-current power grid is judged to be cleared;
2) when the three-phase voltage effective value is smaller than the alternating voltage unbalance fixed value, the fact that the alternating current power grid voltage unbalance fault is cleared is judged;
3) when the angle difference between the negative sequence voltage and the negative sequence current is within the angle difference fixed value range before and after the fault is cleared, judging that the asymmetric fault of the alternating current power grid voltage is cleared;
when the AC power grid positive sequence voltage fault is cleared, the AC power grid voltage unbalance fault is cleared, and the AC power grid voltage asymmetry fault is cleared, the AC power grid fault is judged to be cleared.
4. The method as claimed in claim 3, wherein the converter control method for improving the performance of conventional protection operation of the power system,
in step 2.2, the three-phase line voltage instantaneous values are respectively subtracted from the homonymous line voltage instantaneous values before 20ms, and when the maximum value of the absolute values of the phase deviation is larger than the transient fixed value delta U of the alternating voltage1If so, judging that the voltage of the alternating current power grid is transient, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the AC voltage transient constant value DeltaU1The value range is 0.08 pu-0.15 pu;
step 2.3 comprises:
step 2.3.1, when the positive sequence amplitude of the alternating voltage is less than or equal to the low voltage fixed value delta U2If so, judging that the positive sequence voltage of the alternating current power grid is too low, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the low voltage is constant value delta U2The value range is 0.8 pu-1.0 pu;
step 2.3.2, when the positive sequence amplitude of the alternating voltage is more than or equal to the high voltage fixed value delta U3If so, judging that the positive sequence voltage of the alternating current power grid is too high, otherwise, judging that the voltage of the alternating current power grid is normal;
wherein the high voltage is constant value delta U3The value range is 1.0 pu-1.2 pu;
step 2.3.3, subtracting the effective values of any two line voltages, wherein the absolute value of the difference is larger than the unbalanced constant value delta U of the alternating voltage4Then, it is determined that the AC grid voltage is unbalancedOtherwise, judging that the voltage of the alternating current power grid is normal; wherein the constant value of the AC voltage unbalance is Delta U4The value range is 0.08 pu-0.15 pu.
5. The method as claimed in claim 4, wherein the converter control method for improving the performance of conventional protection operation of the power system,
step 2.4 includes:
step 2.4.1, the positive sequence amplitude of the alternating voltage is larger than the low-voltage fixed value delta U2And less than the high voltage constant value DeltaU3If the state duration is T, the positive sequence voltage fault of the alternating current power grid is judged to be cleared, otherwise, the positive sequence voltage fault of the alternating current power grid is judged not to be cleared; wherein the value range of the state duration T is 10 ms-30 ms;
step 2.4.2, subtracting the effective values of any two line voltages, wherein the maximum value of the absolute value of the difference value does not exceed the set value delta U of the unbalance of the alternating voltage4If the state duration is T, determining that the voltage unbalance fault of the alternating current power grid is cleared, otherwise determining that the positive sequence voltage fault of the alternating current power grid is not cleared; wherein the value range of the state duration T is 10 ms-30 ms;
step 2.4.3, based on the determination result of step 2.4.2, when the angle difference between the negative sequence voltage and the negative sequence current before and after the fault is cleared is within the angle difference fixed value range, it is determined that the asymmetric fault of the alternating current power grid voltage is cleared, and the following relation is satisfied:
90°-x＜∠In-∠Un＜90°+x
in the formula (I), the compound is shown in the specification,
∠Inis the angle of the negative-sequence current,
∠Unis the angle of the negative-sequence voltage,
x is an angle criterion fixed value and the value range is 30-60 degrees.
6. The method as claimed in claim 2, wherein the converter control method for improving the performance of conventional protection operation of the power system,
the step 3 comprises the following steps:
step 3.1, calculating a value by utilizing active powerPcalAnd an active power target value PrefPerforming active power outer loop control on the converter;
step 3.2, utilizing the voltage measurement value U of the direct current sidedcAnd DC side voltage target value UdcrefPerforming outer ring control on the voltage of the direct current side of the converter;
step 3.3, calculating a value Q by utilizing the reactive powercalAnd a reactive power target value QrefPerforming reactive power outer loop control on the converter;
step 3.4, under the normal state of the alternating current power grid, the output current i 'controlled by the active outer ring'dprefAs d-axis positive sequence command current idprefInitial value of (1), output current i 'controlled by reactive power outer loop'qprefAs q-axis positive sequence command current iqprefIs started.
7. The method as claimed in claim 6, wherein the converter control method for improving the performance of conventional protection operation of the power system,
in step 3.1, the converter active power outer loop control meets the following relational expression:
Figure FDA0003372261550000051
in the formula (I), the compound is shown in the specification,
PPIthe output signal of the PI controller, i.e. the active power control value,
Prefin order to be the active power target value,
Pcalthe power is calculated as the active power,
kp1for the scaling factor of the active power closed-loop control,
ki1the integral coefficient is the integral coefficient of active power closed-loop control;
s represents a differential operator;
in step 3.2, the outer ring control of the direct-current side voltage of the converter meets the following relational expression:
Figure FDA0003372261550000052
in the formula (I), the compound is shown in the specification,
UPIthe output signal of the PI controller, i.e. the dc voltage control value,
Udcrefis a target value of the voltage on the direct current side,
Udcis a measurement of the voltage on the dc side,
kp1for the scaling factor of the active power closed-loop control,
ki1the integral coefficient is the integral coefficient of active power closed-loop control;
in step 3.3, the converter reactive power outer loop control meets the following relational expression:
Figure FDA0003372261550000061
in the formula (I), the compound is shown in the specification,
QPIthe output signal of the PI controller, i.e. the reactive power control value,
Qrefin order to be the reactive power target value,
Qcalthe value is calculated for the reactive power,
kp2is a proportionality coefficient of the reactive power closed-loop control,
ki2is the integral coefficient of the reactive power closed-loop control.
8. The method as claimed in claim 6, wherein the converter control method for improving the performance of conventional protection operation of the power system,
step 4 comprises the following steps:
step 4.1, setting the current direction flowing from the power grid to the converter as a positive direction;
step 4.2, in the normal state of the alternating current power grid, if the initial value of the d-axis positive sequence instruction current and the initial value of the q-axis positive sequence instruction current are both smaller than the maximum current I allowed to pass by the power electronic device in the convertermaxOutput current i 'controlled with the active outer loop'dprefAs d-axis positive sequence command current idprefAnd output current i 'controlled by reactive power outer loop'qprefAs q-axis positive sequence command current iqprefAnd applying the dq axis negative sequence command current idnrefAnd iqnrefSetting the current values to be zero, and carrying out closed-loop control on the current of the converter;
if the initial value of the d-axis positive sequence command current or the initial value of the q-axis positive sequence command current is larger than the maximum current I allowed to pass by the power electronic device in the convertermaxThen, the dq-axis positive sequence command current i is calculated in the following relationdprefAnd iqprefAnd applying the dq axis negative sequence command current idnrefAnd iqnrefAll set to zero, carry out closed-loop control to the electric current of converter:
Figure FDA0003372261550000071
wherein σ is the output current i 'controlled by the active outer loop'dprefAnd output current i 'controlled by reactive power outer loop'qprefThe included angle between the composed resultant vectors satisfies
Figure FDA0003372261550000072
4.3, under the fault state of the alternating current power grid, calculating to obtain the fault instruction current of the converter by using the equivalent electromotive force before the fault and the voltage of a grid connection point during the fault; wherein the dq axis positive sequence fault command current idpref0And iqpref0Dq axis negative sequence fault command current idnref0And iqnref0Satisfies the following relation:
Figure FDA0003372261550000073
in the formula (I), the compound is shown in the specification,
udp、uqprespectively the dq-axis component of the positive sequence voltage of the grid-connected point during a fault,
udn、uqnare respectively asThe dq-axis component of the negative sequence of grid-connected points voltage during a fault,
Lcis an equivalent inductance of the current transformer,
erefdp0、erefqp0respectively positive sequence dq axis components of the three-phase modulation wave signals of the power grid,
erefdn0、erefqn0respectively negative sequence dq axis components of the three-phase modulation wave signals of the power grid;
wherein, the three-phase modulation wave signal of the power grid is the three-phase modulation wave signal e of the previous whole period of the current sampling moment recorded and stored in the step 2.1refa0、erefb0、erefc0；
With the calculated dq-axis positive sequence command current idprefAnd iqprefDq axis negative sequence command current idnrefAnd iqnrefCarrying out closed-loop control on the current of the converter;
step 4.4, according to the fault instruction current obtained in the step 4.3, on the premise of keeping the phase of the fault instruction current unchanged, calculating the first coefficient K for reducing the fault instruction current in an equal proportion according to the following relational expressionref1：
Figure FDA0003372261550000074
When calculated Kref1If greater than 1, then K isref1Is set to 1;
calculating the fault command current equal-proportion reduction second coefficient K according to the following relationref2：
Figure FDA0003372261550000081
Pflt＝udpidpref0+uqpiqpref0+udnidnref0+uqniqnref0
In the formula, PnomThe active power output by the converter before the fault; pfltFor the need of converters during faultThe output active power, alpha, is a real number less than 1;
get Kref1、Kref2The minimum value of the fault current is used as the equal proportional reduction coefficient K of the fault command currentrefThe reduced fault command current is calculated according to the following relation:
Figure FDA0003372261550000082
the fault command current after the equal proportion reduction can meet the condition that the fault command current is less than the maximum current I allowed by a power electronic device in the convertermaxThe requirements of (1).
9. The method as claimed in claim 8, wherein the converter control method for improving the performance of conventional protection operation of power system,
the step 5 comprises the following steps:
step 5.1, 2r/3s conversion is carried out on the instruction current or the fault instruction current obtained in the step 4, and a positive sequence abc component i of the instruction current is obtainedparef、ipbref、ipcrefAnd negative sequence abc component inaref、inbref、incref；
Step 5.2, converting the direct current quantity into an alternating current quantity, namely obtaining a reference value i of three phases of the instruction current A, B, C according to the following relational expressionrefa、irefb、irefc：
Figure FDA0003372261550000083
Step 5.3, with command current irefa、irefb、irefcAnd step 1, collecting the voltage u of the grid-connected point of the convertera、ub、ucAnd current ia、ib、icObtaining a command voltage e of the converter based on dead-beat control as an input signalrefa、erefb、erefcThe following relational expression is satisfied:
Figure FDA0003372261550000091
in the formula: t issIs a control period; k is a radical ofDBThe beat-free coefficient is in a range of 0 to 1;
step 5.4, using the command voltage erefa、erefb、erefcThe three-phase duty ratio signal d is calculated in the following relationa、db、dc：
Figure FDA0003372261550000092
In the formula udcThe measured value of the voltage on the direct current side of the converter is obtained;
and 5.5, modulating the three-phase duty ratio signal to obtain trigger pulses of each power electronic device in the converter.
10. The converter control system realized by the converter control method for improving the traditional protection action performance of the power system according to any one of claims 1 to 9,
the system comprises: the power acquisition module, the power control module, the power grid state identification module and the converter control module;
the power acquisition module is used for acquiring the voltage and the current of a grid-connected point of the converter; the device is also used for carrying out positive-negative sequence separation and coordinate transformation on the voltage and the current so as to obtain positive sequence dq axis components and negative sequence dq axis components of the voltage and the current; obtaining an active power calculation value and a reactive power calculation value by utilizing the positive and negative sequence dq axis components;
the power control module is used for outputting an initial value of instruction current controlled by the inner ring of the converter based on the active parameter outer ring control and the reactive parameter outer ring control of the converter by taking the active power calculation value and the reactive power calculation value output by the power acquisition module as input data;
the power grid state identification module is used for judging whether the alternating current power grid is in a fault state or a normal state according to the voltage and the current of the grid-connected point of the converter; when the fault state is judged, whether the fault is cleared or not needs to be judged;
the converter control module is used for respectively calculating the instruction current of the converter in the fault state or the normal state of the alternating current power grid based on the initial value of the instruction current and carrying out closed-loop control on the current of the converter; the control circuit is also used for obtaining the instruction voltage of the converter based on the instruction current, the voltage and the current of the grid-connected point of the converter and based on dead-beat control; and modulating the command voltage to obtain converter control pulses in the fault state or the normal state of the alternating current power grid.
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